The present invention concerns carbon fibers and, more particularly, a process for rendering such fibers sufficiently flexible to permit tortuous mechanical manipulations to be performed on them with little or no mechanical damage. Carbon fibers plasticized according to the present invention possess a higher elongation-at-break.
Carbon fibers find extensive application in the manufacture of "ablative" materials, usually composites made of carbon or graphite tape impregnated with phenolic resin and then formed into the desired shape. In fabric form, these fibers may be used as heating elements in various applications. Carbon or graphite felt is used as a high temperature insulation. Graphite yarn is used to make heat- and corrosion-resistant packing materials.
More recent applications for carbon and graphite fibers are in structural materials in which the carbon filaments are used to reinforce epoxy or other tough resins. Structural materials such as compressor blades for jet engines, aircraft wings, tail or fuselage structures and helicopter rotor blades have been produced from this type of composite.
Composites of the type just described have a high tensile modulus and tensile strength in the fiber direction yet have a relatively low impact strength typical of high modulus, brittle materials. The impact resistance of these composites could be materially improved with only a small loss in modulus and strength if the elongation-at-break could be increased.
Carbon fibers require special care during process operations because the individual filaments in the yarn tend to break easily. The partial breakage of filaments results in a yarn which contains many stray filaments oriented at various angles from the main direction of the yarn. A yarn containing stray filaments has an abraded, hairy appearance rather than that of a smooth coherent bundle. As a yarn containing such stray filaments is processed further, small bits of the stray material are broken off to form an aerosol of very short carbon fibers, sometimes called "fly," whose presence in the manufacturing plant is harmful to personnel and to machinery.
To avoid the breakage of filaments and minimize the production of "fly," the carbon yarn is generally processed more gently than ordinary textile fibers. Typically, it is shaped relatively slowly over large rollers and the lateral direction of travel is not abruptly changed during take-up onto a spool or bobbin. These procedures hinder the economic rate of production and, additionally, yarns or tows of carbon fibers cannot be shaped over sharp bends or easily manipulated into configurations which require tight packing such as in woven cloth. The brittle nature of the carbon fibers limits their use to structures wherein good collimation can be obtained with little mechanical damage. Thus, while it is possible to weave carbon fibers into cloth for use in the preparation of composites such a procedure is inexpedient due to economic factors. Accordingly, in the manufacture of articles containing carbon fibers, it has generally been the practice in the art to fabricate the carbon fiber precursor in the desired form of the final product- by weaving, for example- prior to pyrolysis. However, a process in which a single fiber or yarn could be pyrolyzed and then easily woven into a given form would be desirable because of the relative ease of pyrolysis of a single fiber or yarn precursor as compared to a cloth precursor.
The treatment of carbon fibers with agents which fall within the class of materials which will intercalate single crystals of graphite has been disclosed in the prior art. For example, nitric acid has been used to treat carbon fibers to improve the bonding thereof with the supporting matrix in a composite material. Such a process is disclosed by Scola et al in U.S. Pat. No. 3,660,140. Also, bromine has been used to treat carbon fibers for the purpose of enhancing the tensile strength thereof, as disclosed by Deitz in U.S. Pat. No. 3,931,392. However, nowhere in the prior art has there been reported a general procedure for plasticizing carbon fibers by contacting the fibers with an agent which is capable of intercalating single crystals of graphite.